They're definitely not precise. They consider the brightest thing they see (hopefully but not guaranteed to be the sun) to be an indicator of which way is up and the darkest thing they see (the ground usually but not always) is assumed to be straight down. So using it to keep a plane level probably won't work very well unless you're flying over a grass field with no concrete sidewalks (those confuse it) or reflective surfaces like snow or water, and only at noon when the sun is straight up. They're capable of helping a self-leveling trainer with lots of dihedral flip over when it accidently becomes inverted most of the time, but I wouldn't count on it to actually hold the wings anywhere near level.

And on a side note... Do these things even do anything to level the wings? On a trainer when ya get in trouble you're supposed to reduce throttle and release the sticks so control surfaces go neutral. For some reason I'm thinking that the leveling system in my super cub just replicated that behavior by cutting throttle and stopping me from giving stick input. Certainly not a useful behavior for most planes.